1 . Field of Invention
This invention generally relates to low-noise drive circuits for electroluminescent lamps and electroluminescent lamp assemblies comprising same, and more specifically to a circuit driving an electroluminescent lamp at a frequency at or below the lower threshold of human hearing, thus reducing objectionable noise otherwise generated by such lamps when operated at high frequencies.
2. Description of the Related Art
Electroluminescent lamps are light emitting devices commonly used to provide back lighting in a variety of display applications, including for example wristwatches, portable telephones, remote controls, personal digital assistants, automobile dashboards, avionics instrumentation, and the like.
A typical electroluminescent lamp comprises a thin layer of electroluminescent material--a material that emits light when excited by an electrical field--sandwiched between two thin, electrically conductive electrode layers. At least one of these layers is optically transparent at the wavelength of the electroluminescent emission. By applying a voltage across the two electrode layers, an electric field is induced between them. Tile electroluminescent material is thereby excited and resultantly emits light that is visible through the optically transparent electrode.
The light that is produced by the electroluminescent lamp is essentially uniform across the surface area ("lit area") of its optically transparent electrode. The electroluminescent lamp therefore is ideally suited as a backlight for analog and liquid crystal digital displays.
Due to its structure--two conductive electrodes with a dielectric layer between them--the electroluminescent lamp functionally behaves as a capacitor. The electric field induced by the applied voltage thus decreases as a charge accumulates on one of the electrodes, and correspondingly the light production of the electroluminescent material is attenuated or the electroluminescent material will at charge saturation conditions even cease to emit light.
Alternating current (AC) voltage must accordingly be applied to the electrodes of the electroluminescent lamp to maintain continuous illumination. Typically, AC voltage in the frequency range of 200 Hz to 1000 Hz is used to drive electroluminescent lamps.
Numerous circuits and systems exist within the prior art for driving electroluminescent lamps from low voltage batteries.
U.S. Pat. No. 5,854,539 to Pace, et al. discloses a driving circuit, which produces an alternating current by selectively coupling an inductor to the supply voltage such that current is induced in either of two directions through the inductor. A set of switches then couples the energy that is stored in the inductor across the electroluminescent lamp, alternatively charging it first positively and then negatively in cyclic fashion.
U.S. Pat. No. 5,861,719, to Koskowich, et. al. discloses an electroluminescent lamp driving circuit with a voltage limiting feedback featuring a capacitive voltage divider, to reduce resistive losses and electromagnetic interference.
An inherent problem with electroluminescent lamps is that in operation, such devices emit acoustic energy within the audible range of sound waves, producing an associated objectionable buzz or hum. This noise is particularly problematic in applications such as mobile telephones, which in operation are held close to the user's ear and in which any appreciable noise interferes with the use and operability of the device.
U.S. Pat. No. 5,789,870 to Remson, et al. discloses an approach to reducing the noise generated by electroluminescent lamps. This patent describes a drive circuit, which predicts the transitions from positive to negative charge, and vice versa, and effectively damps these transitions. By increasing the rise and fall times of the voltage pulses applied to the electroluminescent lamp, the transitions of charge at the front and rear electrodes of the lamp are less abrupt. This in turn reduces the rate of physical deformation of the lamp electrodes resulting from opposite electrical charges, and reduces the audible output of the lamp assembly.
The solution described in the Remson et al. patent, however, requires a complex electronic driving circuit, comprising a plurality of operational amplifiers, transistors, diodes, resistors, and capacitors. The component values of these circuit elements must be selected so as to operate properly at each desired frequency.
U.S. Pat. No. 5,566,064 to Schoenwald, et. al. discloses an electroluminescent lamp drive system for eliminating the large magnetic component of resonant inverters operating at low frequencies. This drive system is intended to eliminate the audible energy generated by the mechanical windings and core laminations of the inductor which result from its being driven at frequencies within the human audible range. Schoenwald utilizes a pulse width modulated signal, with the pulse frequency being above the range of human hearing.
The electroluminescent lamp drive system described in the Schoenwald, et. al. patent, however, also requires complex driving circuitry, and addresses only the audible noise that is generated by the inductor. The Schoenwald, et. al. system does not address noise generated from the surface(s) of the electroluminescent lamp itself.
Accordingly, an object of the present invention is to provide a simple, cost-effective driver circuit for electroluminescent lamps which minimizes their objectionable audible output.
Another object of the present invention, based on the fact that the sensitivity of the human ear decreases with frequency below about 1 kHz, is to provide a drive circuit for electroluminescent lamps that operates at a frequency of approximately 60 to 70 Hz, thereby providing an electroluminescent lamp system that has a very low and substantially imperceptible noise output.
Yet another object of the present invention is to provide an efficient electroluminescent lamp assembly in which the electroluminescent lamp is driven at low frequency, but nonetheless provides a high level of illumination output.
A further object of the invention is to provide an electroluminescent lamp operable at a low frequency, by increasing the amplitude of the drive voltage.
A still further object of the invention is to provide an electroluminescent lamp having an extended operational life by virtue of its capability to function at a decreased frequency level.
Other objects, features and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.